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Community Ecology Chapter 56 1 Biological Communities • Community: all the organisms that live together in a specific place – Evolve together – Forage together – Compete – Cooperate 2 Biological Communities • Communities can be characterized either by their constituent species or by their properties – Species richness: the number of species present – Primary productivity: the amount of energy produced • Interactions among members govern many ecological and evolutionary processes 3 Biological Communities • Interactions in a community – Predation – Mutualism • Assemblage: the species included are only a portion of those present in the community 4 Biological Communities • Two views of structure and functioning of communities – Individualistic concept: H.A. Gleason; a community is nothing more than an aggregation of species that happen to occur together at one place – Holistic concept: F.E. Clements: a community is an integrated unit; superorganism-more than the sum of its parts 5 Biological Communities Most ecologists today favor the individualistic concept • In communities, species respond independently to changing environmental conditions • Community composition changes gradually across landscapes 6 Biological Communities • Abundance of tree species along a moisture gradient in the Santa Catalina Mountains of Southeastern Arizona • Each line represents the abundance of a different tree species • Community composition changes continually along the gradient 7 Biological Communities • Sometimes the abundance of species in a community does change geographically in a synchronous pattern • Ecotones: places where the environment changes abruptly 8 Ecological Niche • Niche: the total of all the ways an organism uses the resources of its environment – Space utilization – Food consumption – Temperature range – Appropriate conditions for mating – Requirements for moisture and more Billock 9 Ecological Niche • Interspecific competition: occurs when two species attempt to use the same resource and there is not enough resource to satisfy both • Interference competition: physical interactions over access to resources – Fighting – Defending a territory – Competitive exclusion: displacing an individual from its range 10 Ecological Niche • Fundamental niche: the entire niche that a species is capable of using, based on physiological tolerance limits and resource needs • Realized niche: actual set of environmental conditions, presence or absence of other species, in which the species can establish a stable population 11 Ecological Niche J.H. Connell’s classical study of barnacles 12 Ecological Niche • Other causes of niche restriction – Predator absence or presence • Plant species – Absence of pollinators – Presence of herbivores Billock Billock 13 Ecological Niche Principle of competitive exclusion: if two species are competing for a limited resource, the species that uses the resource more efficiently will eventually eliminate the other locally • G.F. Gause’s classic experiment on competitive exclusion using three Paramecium species shows this principle in action 14 15 Ecological Niche • Niche overlap and coexistence • Competitive exclusion redefined: no two species can occupy the same niche indefinitely when resources are limiting • Species may divide up the resources, this is called resource partitioning • Gause found this occurring with two of his Paramecium species 16 Resource partitioning among sympatric lizard species 17 Ecological Niche • Resource partitioning is often seen in similar species that occupy the same geographic area • Thought to result from the process of natural selection • Character displacement: differences in morphology evident between sympatric species – May play a role in adaptive radiation 18 Ecological Niche Character displacement in Darwin’s finches 19 Ecological Niche • Detection of interspecific competition can be difficult – If resources not limited there may be no competition – Small versus large population size – May be environmental conditions that cause the decline of a species, not competition 20 Ecological Niche • Experimental studies of competition • Seed-eating rodents and Kangaroo rats – 50m x 50m enclosures – Enclosures had openings large enough for seed-eating rodents but not the Kangaroo rats – Monitor the number of small rodents 21 Ecological Niche Detecting interspecific competition 22 Ecological Niche • Interpreting field data: Negative effects of one species on another do not automatically indicate the existence of competition – Adults may prey on juveniles of the other species – Presence of one species may attract predators that prey on both species • Experimental studies are not always feasible 23 Predator-Prey • Predation: consuming of one organism by another • Predation strongly influences prey populations • Prey populations can have explosions and crashes – White-tail deer in Eastern US – Introduction of rats, dogs, cats on islands – New Zealand: Stephen Island wren extinct because of a single cat 24 Predator-Prey Predator-prey in the microscopic world When the prey (Paramecium) are used up in the test tube, the predator (Didinium) also dies 25 Predator-Prey • Introduction of prickly-pear cactus in Australia – The cactus population exploded – Introduced the predator a moth (caterpillar eats the cactus) and the population came under control 26 Predator-Prey • Predation and coevolution – Predation provides strong selective pressure on the prey population – Features that decrease the probability of capture are strongly favored – Predator populations counteradapt to continue eating the prey – Coevolution race may ensue 27 Predator-Prey • Plants adapt to predation (herbivory) by evolving mechanisms to defend themselves – Chemical defenses: secondary compounds • Oils, chemicals to attract predators to eat the herbivores, poison milky sap and others – Herbivores coevolve to continue eating the plants 28 Predator-Prey Insect herbivores well suited to their plant hosts: cabbage butterfly 29 Predator-Prey • Chemical defenses in animals – Monarch butterfly caterpillars feed on milkweed and dogbane families – Monarchs incorporate cardiac glycosides from the plants for protection from predation – Butterflies are eaten by birds, but the Monarch contains the chemical from the milkweed that make the birds sick 30 Predator-Prey Blue Jay learns not to eat Monarch butterflies 31 Predator-Prey Poison-dart frogs of the family Dendrobatidae produce toxic alkaloids in the mucus that covers their brightly colored skin 32 Predator-Prey • Defensive coloration – Insects and other animals that are poisonous use warning coloration – Organisms that lack specific chemical defenses are seldom brightly colored – Camouflage or cryptic coloration help nonpoisonous animals blend with their surroundings – Camouflaged animals do not usually live together in groups 33 Predator-Prey Inchworm caterpillar closely resembles a twig 34 Predator-Prey • Mimicry allows one species to capitalize on defensive strategies of another – Resemble distasteful species that exhibit warning coloration – Mimic gains an advantage by looking like the distasteful model – Batesian mimicry – Müllerian mimicry 35 Predator-Prey • Batesian mimicry – Named for Henry Bates – Discovered palatable insects that resembled brightly colored, distasteful species – Mimics would be avoided by predators because they looked like distasteful species – Feed on plants with toxic chemicals 36 Predator-Prey 37 Predator-Prey • Müllerian mimicry – Fritz Müller – Discovered that several unrelated but poisonous species come to resemble one another – Predator learns quickly to avoid them – Some predators evolve an innate avoidance • Both mimic types must look and act like the dangerous model 38 Predator-Prey 39 Species Interactions • Symbiosis: two or more kinds of organisms interact in more-or-less permanent relationships • All symbiotic relationships carry the potential for coevolution • Three major types of symbiosis – Commensalism – Mutualism – Parasitism 40 Species Interactions • Commensalism benefits one species and is neutral to the other – Spanish moss: an epiphyte hangs from trees 41 Species Interactions • When commensalism may not be commensalism • Oxpickers and grazing animals – Oxpickers eat parasites off of grazers – Sometimes pick scabs and drink blood – Grazers could be unharmed by the insects the oxpickers eat 42 Species Interactions • Oxpickers on an impala • Is it commensalism, parasitism or mutualism ? 43 Species Interactions • Mutualism benefits both species • Coevolution: flowering plants and insects Ants and acacias – Acacias provide hollow thorns and food – Ants provide protection from herbivores 44 Species Interactions • Not all ant and acacia relationships are mutualism • In Kenya, several species of ants live on acacias – One species clips the acacia branches to prevent other ants from living in the tree – Clipping branches sterilizes the tree – A parasitic relationship 45 Species Interactions • Parasitism benefits one species at the expense of another • External parasites: – Ectoparasites: feed on exterior surface of an organism – Parasitoids: insects that lay eggs on living hosts • Wasp, whose larvae feed on the body of the host, killing it 46 Species Interactions External parasite: the yellow vines are the flowering plant dodder, it is a parasite that obtains its food from the host plant it grows on 47 Species Interactions • Internal parasites – Endoparasites: live inside the host – Extreme specialization by the parasite as to which host it invades – Structure of the parasite may be simplified because of where it lives in its host – Many parasites have complex life cycles involving more than one host 48 Species Interactions • Dicrocoelium dendriticum is a flatworm that lives in ants as an intermediate host with cattle as its definitive host • To go from the ant to a cow it changes the behavior of the ant • Causing the ant to climb to the top of a blade of grass to be eaten with the grass 49 Species Interactions • Ecological processes have interactive effects – Predation reduces competition • Predators choice depends partly on relative abundance of the prey options • Superior competitors may be reduced in number by predation • This allows other species to survive when they could have been out competed 50 Species Interactions Starfish eat barnacles, allowing other species to thrive instead of being crowded out by the explosive population of barnacles 51 Species Interactions • Parasitism may counter competition – Parasites may affect sympatric species differently, changing the outcome of interspecific interactions – Flour beetles and a competition experiment • Without a parasite T. castaneum is dominant • With the parasite: T. confusum is dominant 52 Species Interactions • Indirect effects: presence of one species may affect a second by way of interactions with a third species 53 Species Interactions • Keystone species: species whose effects on the composition of communities are greater than one might expect based on their abundance • Sea star predation on barnacles greatly alters the species richness of the marine community • Keystone species can manipulate the environment in ways that create new habitats for other species 54 Species Interactions Beavers construct dams and transform flowing streams into ponds, creating new habitats for many plants and animals 55 Succession and Disturbance • Primary succession: occurs on bare, lifeless substrate – Open water – Rocks • Organisms gradually move into an area and change its nature 56 Succession and Disturbance Primary succession on glacial moraines 57 Succession and Disturbance • Secondary succession: occurs in areas where an existing community has been disturbed but organisms still remain – Example: field left uncultivated – Forest after a fire • Succession happens because species alter the habitat and the resources available in ways that favor other species entering the habitat 58 Succession and Disturbance • Three dynamic concepts in the process – Tolerance: early successional species are characterized by r-selected species tolerant of harsh conditions – Facilitation: early successional species introduce local changes in the habitat. Kselected species replace r-selected species – Inhibition: changes in the habitat caused by one species inhibits the growth of the original species 59 Succession and Disturbance • Animal species in a community can also change over time • Krakatau island – Volcanic eruption – Fauna changed in synchrony with the vegetation – Changes in animals affect plant occurrences; pollination, animal dispersion 60 Succession and Disturbance Succession after a volcanic eruption 61 Succession and Disturbance • Communities are constantly changing as a result of – Climatic changes – Species invasions – Disturbance events • Nonequilibrium models that emphasize change rather than stability are used to study communities and ecosystems 62 Succession and Disturbance • Intermediate disturbance hypothesis: communities experiencing moderate amounts of disturbance will have higher levels of species richness than communities experiencing either little or great amounts of disturbance – Patches of habitat will exist at different successional stages – May prevent communities from reaching the final stages of succession 63 Succession and Disturbance • Disturbance is common, rather than exceptional in many communities • Understanding the role that disturbances play in structuring communities is an important area of ecology 64